The role of oxidative stress in the evolution of diabetic vascular complications is unclear, in part due to limitation of current methodology to assess free radical generation in vivo. The present study will utilize gas chromatography/mass spectrometry (GC/MS) methods for the measurement of two structurally distinct F2-isoprostanes, 8-epi-PGF2a-I. These novel markers of free radical catalyzed lipid peroxidation in vivo will be assessed in conjunction with LDL oxidizability and endothelial vasomotor function to address the hypothesis that oxidative stress is enhanced in diabetes mellitus and that this precedes the onset of micro or macrovascular disease. Urinary excretion of the F2-Isoprostanes will be measured in patients with type 1 diabetes with and without retinopathy; in patients with type 2 diabetes with and without macrovascular disease; in patients with peripheral vascular disease of non-diabetic origin and in appropriate controls. Because isoprostanes are formed in situ from arachidonic acid esterified to phospholipid, we will isolate LDL to measure the degree to which LDL phospholipids contain 8-epi-PGF2a and IPF2a-I. We will also assess the LDL susceptibility to oxidation and the time course of formation of F2-Isoprostanes in the LDL during copper-induced oxidation. Endothelial-dependent and endothelial-independent dilatation of the brachial artery will be measured by high-resolution, non-invasive vascular antioxidants, vitamins E+C, will depress urinary excretion of F2-Isoprostanes concomitantly with modulation of LDL susceptibility to oxidation and restoration of endothelial vasomotor function. The results of this study might support the hypothesis that increased oxidative stress in diabetes precedes the onset of vascular complications and might indicate a therapeutic role for antioxidants in complementing other therapeutic approaches to the prevention or amelioration of diabetic complications.